It is a well known practice to survey a well by acoustic logging techniques in which acoustic signals are generated and received by means of a logging tool run through the well. One such acoustic logging technique involves the generation and reception of acoustic signals and the determination of the travel time of the acoustic signals between a transmitter and a receiver or between spaced receivers. By this technique the velocity of sound through a subsurface formation may be determined in order to characterize the formation.
An acoustic signal may be transmitted through a subsurface formation in the form of both compressional and shear waves. The compressional wave represents acoustic energy which has been refracted through the formation adjacent the wellbore. The compressional wave travels as a fluid pressure wave in the wellbore mud from the transmitter to the formation where it travels at the compressional wave velocity of the particular formation. The compressional wave then travels to the receiver through the wellbore mud as a fluid pressure wave. The shear wave is also refracted through the formation adjacent the wellbore. Unlike the compressional wave, the shear wave travels at shear velocity through the formations.
The velocities of compressional and shear acoustic waves traveling through a formation are dependent on such formation parameters as type of lithology, degree of compaction and cementation, effective overburden stress, porosity and type of saturating fluid. In general, a change in any one of these parameters will cause both compressional and shear velocities to either increase or decrease proportionately. The notable exception to this rule is the acoustic response to gas as part or all of the pore filling fluid of the formation. The introduction of a small amount of gas in the pore spaces causes a large reduction in compressional velocity. At the same time, increasing gas saturation causes a small increase in shear velocity.
Because of these effects on the compressional and shear wave velocities, the result of replacing water with gas in the pore spaces is a large reduction in the ratio of the compressional wave velocity (V.sub.p) to the shear wave velocity (V.sub.s). The magnitude of the reduction is relatively independent of both formation porosity and gas saturation, and is most sensitive to the initial compressibility of the matrix.
It is a specific object of the present invention to utilize the measurement of V.sub.p /V.sub.s in a new and improved way to identify hydrocarbon-bearing zones where the compressional and shear wave velocities vary significantly from water-bearing clastics, i.e. sandstones and shales. Such an objective can be particularly useful in zones where resistivity logs do not provide adequate delineation of hydrocarbon-bearing zones. This would include low resistivity pay zones where resistivities are lower than in normal hydrocarbon-bearing zones due to formation properties and areas with fresh formation waters where high resistivities are common in water-bearing zones. In both cases, resistivity logs may not provide the simple, reliable identification of hydrocarbon-bearing zones that is possible in more typical reservoirs.